2023 Volume 6 Issue 5 (Published 20 August 2024)

Open Access

INEOS OPEN, 2023, 6 (5), 126–137  Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF DOI: 10.32931/io2321r

Catalytic Activity of C,N-Palladacycles in the Suzuki–Miyaura Reaction

Abstract

Owing to the high thermal, oxidative and hydrolytic stability, as well as the ease of structural modification, cyclopalladated complexes (CPCs) have enormous potential in a wide range of chemical reactions and processes. This review highlights the examples of the successful application of azapalladacycles based on various N-donor ligands as the (pre)catalysts for the Suzuki–Miyaura cross-coupling.

Key words: C,N-palladacycles, catalysis, cross-coupling, Suzuki–Miyaura reaction.

 

Introduction

Cross-coupling reactions are one of the most popular methods for constructing C–C bonds and play a huge role in organic synthesis. They are actively used to obtain the analogs of natural compounds, drugs, and agrochemicals [1–5]. It is no coincidence that the 2010 Nobel Prize in Chemistry was jointly awarded to Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki for palladium-catalyzed cross-couplings in organic synthesis [6]. The most popular reaction owing to the synthetic availability and stability of the substrates, namely, arylboronic acids, tolerance towards a large number of functional groups, low toxicity of the reagents and by-products (in contrast to the Stille reaction), predominantly low temperatures (unlike the Heck reaction), a wide scope of substrates, and a minimal amount of the resulting by-products is the Suzuki–Miyaura cross-coupling (SM). For example, the bis-heterocyclic and biphenyl moieties in the popular antihypertensive drugs telmisartan [7] and valsartan [8] were introduced into the molecules using the Suzuki–Miyaura reaction. The same reaction was used to synthesize abemaciclib [9], a drug for the treatment of breast cancer, and atanazavir, a drug for the treatment of infection caused by the human immunodeficiency virus (HIV) [10].

CPCs are a class of organometallic compounds that contain at least one covalent Pd–C bond stabilized by an additional coordination bond with a heterodonor (N, S, O, P). Due to their thermal and oxidative stability, as well as ample opportunities for structural modification, these compounds have enormous potential in a wide variety of chemical reactions and processes.

CPCs act as the key intermediates in heterodonor-directed C–H functionalization of organic substrates, display luminescent and liquid crystalline properties [11], exhibit a broad scope of biological activity, including antiparasitic [12], antibacterial and antioxidant [13], antifungal and antimicrobial [14, 15], and antimalarial [16] properties, as well as high antitumor activity [17–20]. Optically active CPCs are used as the matrices for the resolution of racemic substrates capable of coordinating with the palladium atom [11], as well as the catalysts in enantioselective catalysis [21].

Traditionally, commercially available reagents such as Pd(OAc)₂ and Pd(PPh₃)₄ are used as the  catalysts in cross-couplings, which are quite effective catalysts in the presence of P- and N-donor ligands. However, the unique catalytic activity of phosphapalladacycles in cross-couplings, discovered in 1995 by Herman, opened a new page in achiral homogeneous catalysis [22–24]. Since then, the catalytic activity of dozens of new types of palladacycles has been studied, but interest in this field is only growing [24, 25]. The success and efficiency of cross-coupling reactions largely depend on the catalytic system in use, so even small changes in the catalyst structure can lead to increased activity and selectivity, and, as a result, to economic benefits.

Recently, there has been increased interest in the use of palladacycles based on N-donor ligands as the catalysts for cross-couplings. This is stipulated by the much higher stability of the nitrogen-containing ligands compared to the P-donor counterparts, as well as their lower cost and toxicity [26].

The goal of this review was to analyze the modern data on the catalytic activity of C,N-palladacycles in the Suzuki–Miyaura cross-coupling, focusing on the publications from 2017 to 2023. The earlier reports on the use of azapalladacycles as cross-coupling (pre)catalysts, including enantioselective modifications, were summarized in other reviews [21, 24, 27–31].

2. C,N-Palladacylces based on amines

Nowadays, amine-based palladacycles comprise a broad group of CPCs, consisting mainly of the complexes with an sp²-metalated carbon atom [30]. Aminate CPCs, owing to their high catalytic activity, are currently extensively used in various C–C bond forming reactions, including the Heck, Suzuki-Miyaura and Sonogashira cross-couplings [24, 25]. It should be noted that the catalytic activity of mononuclear derivatives has been mainly explored, while the dimers themselves are practically not used as the (pre)catalysts.

Karami et al. [26] demonstrated the effectiveness of a series of C,N-palladacycles 1–4 (Fig. 1) based on secondary amines in the SM reaction between aryl bromides and PhB(OH)₂.

Figure 1. Derivatives of benzylaminate C,N-palladacycles.

In the cross-coupling of bromobenzene with PhB(OH)₂, the mononuclear derivatives with both monodentate pyridine ligands (1, 2) and bidentately bound diphosphines (3–5) showed high catalytic activity: the product yields were 100% and 86–92%, respectively [26]. However, in the case of coordinating substrates, such as 2-bromopyridine, a stronger contrast was observed (Scheme 1). When the reaction was catalyzed by mononuclear complexes 1, 2, the coupling product was formed in a quantitative yield, while in the case of cationic complexes 3–5 featuring bidentately bound diphosphines 1,2-bis(diphenylphosphino)ethane (dppe) and 1,3-bis(diphenylphosphino)propane (dppp), the yield of the target product appeared to be significantly lower (21–30%).

Scheme 1. Suzuki–Miyaura cross-coupling between 2-bromopyridine and PhB(OH)₂.

A comparison of the catalytic performance of phosphine derivatives 3 and 5 revealed that complex 3 exhibits slightly higher activity than its less sterically hindered analog. Thus, the yields of the coupling product in the reaction of bromobenzene with PhB(OH)₂ catalyzed by these complexes were 93% and 86%, respectively. The authors noted that CPC 6 bearing the bridging diphosphine ligand exhibits considerably higher catalytic activity than its counterpart 5 containing the same but bidentately coordinated ligand: when 2-bromopyridine reacted with PhB(OH)₂, the product yield was 75 % and 30% in the case of azapalladacycles 6 and 5, respectively [26]. However, this comparison is not exactly correct, since the molecule of complex 6 contains two palladium atoms, and the authors did not take this into account when calculating the catalyst loadings.

Poly(ethylene glycol) (PEG), an unconventional solvent for the SM cross-coupling, was used to study the catalytic activity of CPC 7 (Fig. 2) based on primary benzylamine with a bidentate coordinated P,O-ligand dppmo (dppmo = Ph₂P(CH₂)P(O)Ph₂) [32].

Figure 2. Derivatives of benzylaminate C,N-palladacycles.

A number of aryl iodides and aryl bromides reacted effectively with PhB(OH)₂ at the catalyst loading of 0.01 mol % over a short period of time (20–40 min), although the reactions were performed at elevated temperatures. In all cases, the products were obtained in high yields (89–97%). Furthermore, complex 7 turned out to be effective even in the cross-coupling of aryl chlorides. The yields of the coupling products in this case ranged within 89–92% (Scheme 2).

Scheme 2. Suzuki–Miyaura reaction catalyzed by complex 7.

Recently, our research group demonstrated the high catalytic activity of benzylaminate dimeric C,N-palladacycles in the Suzuki–Miyaura reaction between aryl bromides and PhB(OH)₂ under mild conditions [33]. It should be noted that this is one of the rare examples of the application of dimers as (pre)catalysts for this process. The highest catalytic activity at 0.1 mol % [Pd] was observed in the case of dimer 8, bearing a tert-butyl substituent at the α-carbon atom and a primary amino group, and complex 9, containing a phenyl moiety and a tertiary amino group (Fig. 2, Scheme 3). As the concentration of (pre)catalyst 9 was reduced to to 0.01 mol % [Pd], the efficiency of the process was maintained by increasing the reaction temperature (up to 65 °C). In this case, the coupling product was formed in 94% yield in 2 h. Moreover, dimer 9 catalyzed the reaction even at the lower loadings (up to 0.001 mol %), but the formation of 4-methoxybiphenyl in a high yield (81%) requires a longer time and heating (15 h at 65 °C).

Scheme 3. Suzuki–Miyaura reaction catalyzed by complex 9.

CPCs containing four palladium centers are rarely used in catalysis. However, the advantage of multinuclear complexes is the presence of multiple catalytically active metal centers in a relatively small organic ligand. Thus, tetranuclear cyclopalladated (pre)catalyst 10 (Fig. 2) at a loading of only 0.0002 mol % showed high efficiency in the reaction between 4-bromoacetophenone and PhB(OH)₂, affording the target product in 97% yield. The authors suggested that during the reaction, these tetranuclear palladium(II) complexes disintegrate into two halves, and the resulting binuclear fragments are capable of generating catalytically active forms of palladium at both ends of the resulting molecules [34].

Recently, the collection of aromatic bidentate P-donor ligands used in the Suzuki–Miyaura cross-coupling was expanded by two new sterically hindered phosphines, namely, HandaPhos [35] and EvanPhos [36]. The use of these ligands ensures a significant reduction of the concentration of a palladium catalyst, as well as the reactions under conditions of aqueous micellar catalysis. An environmentally benign surfactant (TPGS-750-M), a diester consisting of racemic α-tocopherol, MPEG-750, and succinic acid, was used to form nanomicelles.

The mononuclear palladacyclic derivatives based on 2-phenylaniline (the so-called Buchwald catalyst) containing the bulky phosphine ligand HandaPhos (Fig. 3) were tested in the SM cross-coupling of various aryl bromides [37].

Figure 3. Phosphine derivatives of the C,N-palladacycle based on 2-phenylaniline.

Diisopropyl-substituted phosphine complex 11 appeared to be the most effective catalyst. In the SM reaction of bulky 5-bromo-meta-xylene and 4-tert-butylphenylboronic acid under mild conditions, with the low catalyst loading, an excellent product yield of 99% was achieved (Scheme 4).

Scheme 4. SM reaction between 5-bromo-meta-xylene and 4-tert-butyl-phenylboronic acid.

The effectiveness of catalyst 11 was demonstrated on a great variety of aryl halides containing both electron-donating and electron-withdrawing groups. In addition, the substrates and arylboronic acids based on heterocycles, the substrates with protecting groups and polyaromatic compounds were involved in the reaction. Using CPC 11 as the (pre)catalyst, a multi-stage one-pot synthesis of the fungicide boscalid as well as drugs anacetrapib, sonidegib, and valsartan was accomplished. The main drawback of this approach, according to the authors, is the laborious synthesis of HandaPhos [35].

Later, the same researchers obtained a series of new mononuclear C,N-palladacycles based on biphenylamines with another more accessible monophosphine ligand EvanPhos [38]. In the reaction between substituted 5-bromopyrimidine and 4-chlorophenylboronic acid, catalyst 12 (Fig. 3), containing an isopropyl group both in one of the phenyl rings and on the nitrogen atom (Scheme 5), was found to be the most effective complex.

Scheme 5. SM reaction catalyzed by C,N-palladacycle 12.

The effectiveness of complex 12 at low concentrations under conditions of aqueous micellar catalysis was demonstrated on a large number of the examples. Thus, it successfully catalyzes the coupling of the substrates containing various functional groups (esters, nitriles, aldehydes, amides, amines, nitro compounds, and carbamates), thiophene-based halides, alkenes and alkynes. A series of heteroaryl chlorides were also reacted, and in each case the coupling product was obtained in good yield. In addition, the possibility of using complex 12 for the synthesis of drugs was shown. Using this catalyst, for example, intermediates for the synthesis of valsartan, sonidegib, anacetrapib, anticancer agents, and an agonist of α2- and α3-GABA receptors were obtained in high yields (83–93%) (Scheme 6).

Scheme 6. Synthesis of an intermediate of an agonist of α2-, α3-GABA receptors.

In recent years, reactions performed under microfluidic conditions have become very popular [39]. Sieber et al. [40] successfully used this approach for the Suzuki–Miyaura cross-coupling between 2-chloropyridine and arylboronic acid catalyzed by palladacycle 13 (Fig. 4, Scheme 7).

Figure 4. Phosphine derivatives of C,N-palladacycles.

Scheme 7. SM reaction under microfluidic conditions catalyzed by palladacycle 13.

The authors noted that performing the reactions in a microreactor ensures an increase in the yield of the target product, a reduction in the reaction time, scaling up to gram quantities, and avoiding the catalyst deactivation, which is observed in traditional catalysis.

One aspect of expanding the application of the Suzuki–Miyaura reaction is the search for conditions that allow the use of inexpensive and readily available aryl chlorides as substrates as well as water as an ecologically friendly solvent. The water-soluble arylpyrazine palladacycle 14 (Fig. 4) with the bulky phosphine auxiliary SPhos was suggested as an effective catalyst for the SM reaction of aryl chlorides [41]. In the reaction of aryl chlorides containing both electron-donating and electron-withdrawing substituents with (hydroxymethylphenyl)boronic acid catalyzed by complex 14, the coupling products were formed in high yields of 88–99% (Scheme 8). The catalyst loading was only 0.2 mol % at that.

Scheme 8. SM reaction catalyzed by azapalladacycle 14.

Benzo[h]quinolinate palladacycle 15 (Fig. 4) stabilized with a phosphine ylide ligand exhibited high catalytic activity in the cross-coupling of various aryl halides with PhB(OH)₂ [42]. As it was expected, the use of aryl bromides as substrates enabled the production of the target biphenyls in high yields (89–98%) over a short period of time (10–25 min), albeit upon heating (Scheme 9). On passing to aryl chlorides, the reaction time increased (40–60 mins) and the yields of the target products decreased (60–80%).

Scheme 9. SM reaction catalyzed by azapalladacycle 15.

Firinci [43] showed high catalytic activity of three CPCs 16–18 based on 2-phenylpyridine containing a barbiturate ligand in the SM cross-coupling (Fig. 5). Thus, the reactions of 4-bromotoluene with PhB(OH)₂ promoted by 1 mol % of catalysts 16–18 afforded the final products in 1–4 h in good yields of 76–99%. When 4-bromoacetophenone was used as a substrate, the reaction time was halved.

Figure 5. C,N-Palladacycles based on 2-phenylpyridine.

The catalytic properties of aminate C,N-palladacycles can be improved by complex depositing on a polymer substrate. The latter can be graphene oxide [44]. A two-dimensional structure with huge surface area as well as the presence of chemically active sites for functionalization make it a promising carrier for nanoparticles and complexes.

Palladacycle 21 immobilized on graphene oxide in the reaction of aryl bromides with PhB(OH)₂ under mild conditions (toluene, K₂CO₃, 20 °C, ~0.001 mol %) showed catalytic activity comparable with that of homogeneous analogs 19 and 20 and provided the target products in good yields (70–90%) (Fig. 5). At the same time, its efficiency in the cross-coupling of aryl chlorides was significantly higher than that of palladacycles 19 and 20. For example, in the reaction of chlorobenzene with PhB(OH)₂ catalyzed by complex 21 under mild conditions, the coupling product was formed in 67% yield in just 45 min, whereas in the presence of complexes 19 and 20, the reaction time reached 2–3 h. An obvious advantage of heterogeneous catalyst 21 immobilized on the graphene substrate is the ease of its separation from the reaction mixture as well as the possibility of recycling.

Recent decades have witness the growth in the popularity of C,N-palladacycles bearing an additional N-heterocyclic carbene ligand in the SM reaction. Carbenes constitute a class of ecologically friendly ligands with unique electronic and steric properties [45, 46]. A series of C,N-palladacycles based on 2-phenylpyridine 21–23 [47] and tertiary benzylamines 24–26 [47], 27 [48] with N-butyl-substituted N-heterocyclic carbenes as auxiliary ligands (Fig. 6) were tested in the SM cross-coupling.

Figure 6. Mononuclear carbene derivatives of C,N-palladacycles.

All the complexes (at 1 mol % loading) showed high performance in the SM reaction. Palladacycles 24–26 appeared to be slightly less active than catalysts 21–23: in the reaction of 4-bromotoluene with phenylboronic acid, the yields of the coupling products were 63–92% and 93–99%, respectively. It should be noted that the activity of the complexes increased slightly with the increasing number of methyl groups in the benzyl carbene substituent. In addition, complexes 23 and 26 were tested in the SM reaction of aryl chlorides. Thus, in the coupling between 4-chloroacetophenone and PhB(OH)₂ catalyzed by these palladacycles, the target products were obtained in moderate yields of 43% and 45%, respectively. The authors indicated that catalysts 21–26 with an n-butyl substituent are more efficient than the previously described related complexes containing methyl groups [49].

Chinese scientists demonstrated high catalytic activity of mononuclear derivative 27, a benzylaminate palladacycle with an alkoxy-N-heterocyclic carbene ligand, in the SM cross-coupling with various aryl chlorides (Fig. 6, Scheme 10) [48].

Scheme 10. SM reaction of aryl chlorides catalyzed by palladacycle 27.

Substituents in aryl chlorides (OMe, CHO, C(O)Me, CN, and NO₂) have virtually no effect on the yields of the reaction products (71–93%). In addition, heteroaryl chlorides, for example, 3-chlorothiophene, also entered the SM reaction catalyzed by complex 27, forming the coupling products in good yields (Scheme 11).

Scheme 11. SM reaction between 3-chlorothiophene and PhB(OH)₂ catalyzed by palladacycle 27.

The same research group compared the catalytic activity of CPC 28, featuring a rigidly fixed aliphatic spacer between the imidazolium ligand and the palladated phenylene moiety, with its analog 29, bearing more flexible alkyl substituents, in the SM cross-coupling of various aryl chlorides in an aqueous medium (Fig. 7) [50].

Figure 7. Mononuclear carbene derivatives of benzylaminate C,N-palladacycles.

In the model reactions of chlorobenzene with 4-methoxyphenylboronic acid acid under similar conditions (0.5 mol %, K₂CO₃, EtOH/H₂O, 50 °C, 6 h) in the presence of catalysts 28 and 29, 4-methoxybiphenyl was obtained in 95% and 63% yield, respectively. Catalyst 28 exhibited high performance in the reactions with aryl halides, bearing both electron-donating and electron-withdrawing substituents, as well as with sterically loaded substrates and heterocycles. These results indicate that rigid complex 28 has better catalytic activity than its more flexible analog 29, which should be taken into account when developing new potential catalysts.

Recently, the high catalytic activity of an acenaphthoimidazolylidene derivative of C,N-palladacycle 30 in the SM cross-coupling of inactive amides with various aryl-, alkyl-, and alkenylboronic acids was demonstrated to form the corresponding ketones in high yields [51]. Thus, benzylamide with N-Bn and electron-withdrawing bulky N-Boc groups in the reaction with PhB(OH)₂ catalyzed by CPC 30 was almost quantitatively converted into the corresponding ketone (Scheme 12):

Scheme 12. Synthesis of ketones using the SM reaction catalyzed by complex 30.

The catalytic activity of complex 31 was significantly lower than that of catalyst 30. For example, in the reaction between an inactive benzylamide with N-Ts/Bn substituents and PhB(OH)₂ in the presence of 3 mol % of the catalyst, the corresponding ketone was formed in 63% yield for palladacycle 30 and only 15% in the case of complex 31. The authors explain this by the strong σ-donor and weak π-acceptor properties of acenaphthoimidazolylidene, as well as an increase in the size of the ligand π-ring in complex 30.

3. Palladacycles based on oximes

Oximate palladacycles serve as efficient and versatile (pre)catalysts for a wide range of cross-couplings in air and at low catalyst loadings [29]. These palladacycles are highly stable to the action of oxygen air, moisture, as well as heat and can be obtained from inexpensive precursors.

The derivatives of the simplest oximate palladacycle with P,P-bidentately bound dppp 32 [52] and an unsymmetrical phosphorus ylide based on acetophenone 33 [53, 54], as well as complex 34 [55] bearing both a bridged and bidentate diphosphine dppe ligands were suggested as the catalysts for the SM coupling of aryl bromides with phenylboronic acid (Fig. 8).

Figure 8. Derivatives of the oximate C,N-palladacycle with P-donor ligands.

It was shown that complexes 32 and 33 at ~0.5 mol % loading effectively promoted the cross-coupling of aryl bromides with PhB(OH)₂ under mild conditions, leading to substituted biaryls in high yields (Scheme 13).

Scheme 13. Catalytic activity of complexes 32 and 33 in the SM reaction.

The high catalytic activity was also observed in the case of binuclear oximate palladacycle 34 [55]. Thus, the reaction of 4-bromoanisole with PhB(OH)₂ catalyzed by 0.5 mol % [Pd] of complex 34 afforded the coupling product in 96% yield in 2 h.

The popularity of cross-coupling catalysts based on oximate palladacycles immobilized on various supports, such as SiO₂, ionic liquids, resins, and polymer substrates, is stipulated by the ease of separation and the possibility of catalyst recycling [29, 56–59]. However, significant disadvantages of such systems are an increase in the temperature and duration of the process, as well as the presence of an induction period, since the precatalyst slowly converts to an active form. This problem was solved by performing the reactions under microwave irradiation [60].

Thus, trialkoxy-substituted oximate palladacycle 35 (Fig. 9) immobilized on a polymer substrate in the reaction of aryl bromides with PhB(OH)₂ at a loading of 1 mol % provided biaryl products in high yields (96–99%) in just 30 min (Scheme 14). However, in the reactions of the corresponding aryl chlorides, a considerable decrease in the yields of the target products was observed (20–53%). When comparing the kinetic profiles of the cross-coupling between para-bromoanisole and PhB(OH)₂ under heating and microwave irradiation, a significant decrease in the induction period from 20 to 3 min was detected and the reaction with microwave activation completed in 20 min. Moreover, catalyst 35 was reused in five consecutive cycles without any loss in the catalytic activity with excellent coupling yields (>94%).

Figure 9. Heterogeneous oximate C,N-palladacycles.

Scheme 14. Catalytic activity of complex 35 in the SM reaction.

The high catalytic activity in the Suzuki–Miyaura reaction was demonstrated by oximate palladacycle 36 immobilized on graphene oxide (Fig. 9). The cross-coupling of aryl bromides was carried out in water at room temperature and at catalyst loadings reduced to 0.002 mol % [Pd] [61]. Catalyst 36 can be recovered and reused with loss in the catalytic activity after the second cycle due to metal oxidation and agglomeration processes.

4. C,N-Palladacycles based on imines

Iminate CPCs are known to be effective catalysts for cross-coupling reactions [27, 30]. Like most palladacycles, they can potentially be converted to a wide range of functional derivatives.

Babahan et al. [62] studied the catalytic activity in the SM reaction of a series of iminate C,N-palladacycles: μ-acetate dimers 37 and 38 and their mononuclear derivatives with carbene ligands 39–44 (Fig. 10).

Figure 10. Dimeric iminate C,N-palladacycles and their derivatives with carbene ligands.

The catalytic activity of these complexes in the SM reaction between aryl bromides and PhB(OH)₂ was shown to depend on the structures of both the substrate and the catalyst itself (Scheme 15).

Scheme 15. Catalytic activity of complexes 39–44 in the SM reaction.

For example, in the case of electron-neutral bromobenzene, the yields of the coupling products were 48–98%. As expected, 4-bromoacetophenone was more reactive, while the reactions between electron-rich 4-bromotoluene and PhB(OH)₂ resulted in the coupling products in poor to moderate yields (10–45%). When comparing the catalytic activity of CPCs 37–44, it was found that acetate-bridged dimers 37 and 38 were more active than carbene-containing mononuclear derivatives 39–44. Thus, in the reaction of bromobenzene with PhB(OH)₂ catalyzed by complexes 37 and 38, the product yields were 98% in each case, while in the case of complexes 39–44, they ranged within 48–98%. Dimer 37 appeared to be the most active catalyst in the Suzuki–Miyaura reaction (98–99% yields in all three reactions).

A large series of iminate C,N-palladacycles were studied as the catalysts for the Suzuki–Miyaura cross-coupling by Bermúdez-Puente et al. [63]. The presence of methoxy substituents in the phenylene ring increased the solubility of these palladacycles, allowing catalysis to be carried out in an aqueous medium.

Dimers 45–50 as well as their derivatives with bipyridine 51, 52, 1,10-phenanthroline 53, 54, triphenylphosphine 55, 56, acetylacetonate 57, 58, and complexes with bridged 59, 60 and chelating diphosphine 61, 62 (Fig. 11) were tested in the model SM cross-coupling between 4-bromoacetophenone and PhB(OH)₂ at room temperature and 80 °C (Scheme 16) [63].

Figure 11. Iminate C,N-palladacycles 45–62.

Scheme 16. Catalytic activity of iminate C,N-palladacycles 45–62 in the SM reaction.

In the presence of 2 mol % of the catalyst, the product was obtained in most cases (Scheme 16) in >80% yields.

The derivatives of azapalladacycles with a 1,10-phenanthroline ligand (complexes 53, 54) showed low performance. Catalysis was found to be possible only at elevated temperatures (80 °C), and the product yields were only 20% and 33%. In the case of the SM reactions catalyzed by palladacyclic derivatives with bipyridine 51, 52, heating and a long time (24 h) were required to obtain good yields of the substituted biphenyl (100% and 66%).

It should be noted that when dimers 48–50 and mononuclear triphenylphosphine adducts 55, 56 were used as the catalysts, high yields of the coupling products (80–100%) were observed both at room temperature and upon heating.

Bermúdez-Puente et al. [63] studied the catalytic activity of (μ)-chloride dimer 49 in more detail due to its stability and ease of synthesis. In the reactions of aryl bromides with both activating (-CHO, -C(O)Me) and deactivating (-OMe) substituents catalyzed by complex 49 under mild conditions, the product yield was 100%. The reaction with para-chloroacetophenone took longer to complete (24 h, 98%), but the reaction with aryl chloride containing a deactivating (-OMe) substituent afforded the target product in only 13% yield.

For catalysts 59, 60 bearing a bridged diphosphine ligand, high yields of the target products (95–99%) were achieved at room temperature, but the reaction time increased to 24 h. Heating to 80 °C allowed for reducing the reaction time to 5 h. Complexes with a bidentate coordinated diphosphine 61, 62 and acetylacetonate 57, 58 were also effective at elevated temperatures (90–95%).

Cyclopalladated iminate dimers 63–65 as well as their triphenylphosphine derivatives 66–68 (Fig. 12) were used as the catalysts for modifying nucleosides, which are classified as bioactive molecules [64]. The functionalized nucleosides are widely used as biological or fluorescent probes as well as in antiviral and antitumor therapy [65, 66].

Figure 12. Iminate C,N-palladacycles 63–68.

In the cross-coupling of 5-iodo-2'-deoxyuridine with benzofuran-2-boronic acid in an aqueous medium (Scheme 17), dimers 63–65 showed higher catalytic activity than their phosphine derivatives 66–68: the product yields ranged within 81–90% and 72–83%, respectively. Complex 65 showed the best performance, presumably due to the better solubility compared to dimers 63 and 64.

Scheme 17. Microfluidic synthesis of a substituted deoxyuridine catalyzed by complex 65.

The coupling of 5-iodo-2'-deoxyuridine with a series of substituted arylboronic acids in the presence of 0.5 mol % of complex 65 was also explored and shown to provide the biaryl products in good yields (68–87%). Only in the case of bulk phenanthreneboronic acid, a decrease in the product yield to 48% was observed.

Recently, the same research group developed the conditions for modifying substituted pyrimidine nucleosides by the SM reaction catalyzed by palladacycle 65 under microfluidic conditions [68]. One of the main obstacles in flow synthesis is the non-homogeneity of reaction mixtures, which can lead to the system blockage; however, the good solubility of dimer 65 in an aqueous medium allowed for avoiding this problem. Thus, the cross-coupling between 5-iodo-2'-deoxycytidine and 3-methoxyphenylboronic acid under traditional conditions of the SM reactions (1 mol %, EtOH/H₂O, K₃PO₄, 60 °C, 24 h, N₂) provided, along with the target biaryl (76%), the product of dehalogenation (14%), and in the absence of an inert atmosphere the reaction did not proceed at all. When passing to the flow catalytic cross-coupling of 5-iodo-2'-deoxycytidine with 3-methoxyphenylboronic acid, the reaction proceeded selectively to form only the target product in high yield in just 30 min (Scheme 18).

Scheme 18. Microfluidic synthesis of a substituted deoxycytidine catalyzed by complex 65.

Hence, a series of aryl-substituted nucleosides was obtained in high yields (74–90%), independent of the substituents in the arylboronic acid or the nucleophilicity of the reagents in use. The main advantages of this approach include a considerable reduction in the reaction time, the absence of an inert atmosphere, the possibility of scaling, high yields of the coupling products, and the ease of isolating the reaction products without recourse to column chromatography.

The catalytic activity and enantioselectivity of the non-metallocene planar chiral iminate C,N-palladacycle (R_pl)-69 was evaluated in the model atroposelective reaction of 1-naphthylboronic acid with 2-methoxynaphthyl-1-iodide [69] (Scheme 19).

Scheme 19. Atroposelective SM reaction catalyzed by C,N-palladacycle (R_pl)-69.

Under mild conditions, 2-methoxy-1,1'-binaphthyl was formed in high yield and with moderate enantioselectivity. An increase in the catalyst loading to 5 mol % and reaction time led to an increase in the enantiomeric purity of the target binaphthyl to 53% ee. It should be noted that this reaction is one of the rare examples of the application of the enantioselective Suzuki–Miyaura cross-coupling catalyzed by palladacycles.

5. Unsymmetrical pincer CPCs

Pincer palladacycles comprise a class of complexes containing at least one Pd–C bond and two intramolecular coordination bonds with heteroatoms [70]. They are the objects of intensive research owing to the high stability, the possibility of structural modification, and the prominent catalytic activity [71].

Unsymmetrical pincer palladacycle 70 (Fig. 13), containing a hybrid C,N,N-tridentate ligand based on a hydrazone and a thiazolyl group, was tested in the reaction of aryl halides with various arylboronic acids in water under IR irradiation [72].

Figure 13. Pincer-type C,N,N-palladacycles 70–73.

The reaction between 4-bromoanisole and PhB(OH)₂ catalyzed by palladacycle 70 afforded the target product in 45% yield in 60 min; the addition of TBAB to the reaction mixture led to an increase in the biaryl yield to 90% (Scheme 20).

Scheme 20. Catalytic activity of complex 70 in the SM reaction.

Further studies on the catalytic activity of palladacycle 70 showed that aryl bromides with both electron-withdrawing and electron-donating groups can react with PhB(OH)₂ in a short period of time (30–90 min) to form the coupling products in high yields (90–96%). In the reactions with sterically hindered substrates, such as 3-bromotoluene, 2-bromotoluene, 2-bromo-1,3-dimethylbenzene, high yields (85–93%) of the corresponding coupling products were achieved by increasing the process duration. Heterocyclic substrates such as 2-bromopyridine, 3-bromopyridine, 2-bromothiophene were also introduced into the SM reaction to form the corresponding biaryls in moderate yields (50–60%), although over longer reaction times. Various arylboronic acids featuring methyl, methoxy, nitro, and trifluoromethyl substituents successfully reacted with 4-bromoanisole with high product yields (70–95%). The reactions with aryl chlorides required a higher catalyst loading (0.1 mol %). While excellent yields (95%) were obtained with activated aryl chlorides, deactivated substrates such as 4-chlorotoluene and 4-chloroanisole afforded low yields (10% and 15%, respectively) [72].

Unusual unsymmetrical C,N,S-pincer complexes 71–73 based on N,N',N"-tris(2-thioanisyl)guanidine, containing a nucleophilic SMe group in the ligand (Fig. 13), were tested in the Suzuki–Miyaura reaction [73]. It was shown that palladacycles 71 and 72 with alkoxy anions showed higher catalytic performance than bromide analog 73: in the model reaction between para-bromoanisole and PhB(OH)₂, when the catalyst loadings were 0.1 mol %, the cross-coupling product was formed in 95–98% and 81%, respectively. Subsequently, complex 71, as the most active one, was tested in the reaction of various aryl bromides with PhB(OH)₂ under similar conditions (Scheme 21).

Scheme 21. Catalytic activity of complex 71 in the SM reaction.

The corresponding cross-coupling products were formed in 65–98% yields. The best result was achieved in the reaction with para-bromoacetophenone (98%), while the worst result—with para-(4-bromophenyl)acetophenone (65%). CPC 71 was also tested in the coupling of para-bromoanisole with a series of substituted arylboronic acids. Depending on the structure of the boron reagent, the product yields ranged within 63–92%.

Unsymmetrical 5,6-membered C,N,N-pincer complexes based on bicyclo[3.3.l]nonane and bicyclo[3,2.l]octane, featuring a phenylene (74, 75) [70] or ferrocenyl (76 [74], 77 [75]) backbone, showed high catalytic activity in the cross-coupling of aryl bromides with PhB(OH)₂ (Fig. 14).

Figure 14. Unsymmetrical 5,6-membered C,N,N-palladacycles.

In all reactions catalyzed by (pre)catalysts 74–77 (0.1–0.5 mol %), the corresponding biaryls (R = Me, OMe, C(O)Me) were formed in high yields of 81–99% (Scheme 22). Using the reaction of 4-bromoanisole with PhB(OH)₂ as an example, it was shown that a reduction in the catalyst loading to 0.05 mol % leads to a significant decrease in the yields of the target products to 9–13%.

Scheme 22. Catalytic activity of complexes 74–77 in the SM reaction.

Bulygina et al. [70] stated that pincer CPCs 74 and 75 with two sp³-hybridized nitrogen atoms are slightly more active catalysts for this reaction than unsymmetrical C,N,N-palladacycles with two sp²-hybridized nitrogen atoms [76]. It should be noted that complex 77 was also effective at room temperature (74–90%) [75]. The authors conclude that neither the presence of the ferrocenyl moiety nor the different structures of the bicyclic ligand have a noticeable effect on the catalytic activity of complexes 74–77.

6. C,N-Palladacycles based on ferrocenes

Active (pre)catalysts for the Suzuki–Miyaura, Heck, Sonogashira, and Buchwald–Hartwig reactions are not only the complexes in which the Pd atom is bound with an aromatic ring, but also the bimetallic derivatives in which Pd is attached to an organometallic moiety, in particular, a ferrocenyl core. The known ferrocenyl palladacycles based on tertiary amines [77, 78], quinolone [79], Schiff bases [80], 3-pyridyl bromide [81, 82], pyridazine [83], pyrimidine [84], and other ligands are effective catalysts for the SM reaction.

In a recent work, Bulygina et al. [85] studied the catalytic activity of new six-membered C,N-palladacycles based on 1-N-ferrocenylmethylindazole 78–85 in the SM reaction (Fig. 15).

Figure 15. Ferrocene-based C,N-palladacycles.

The reaction of 4-bromoanisole with PhB(OH)₂ in the presence of catalyst 78 at room temperature (0.1 mol %, K₂CO₃, MeOH/H₂O, 5 h) afforded the coupling product in 77% yield. Under the same conditions, the coupling of PhB(OH)₂ with 4-bromotoluene resulted in the target biaryl only in moderate yield (69%), while analogous reaction with 4-bromoacetophenone, as expected, led to the corresponding product in high yield (92%). When the reaction temperature was raised to 55 °C, all three aryl bromides were converted to the corresponding biaryls in high yields (89–95%). The results of the investigations on the catalytic activity of complex 78 are comparable with the data for other ferrocenyl CPCs reported in the literature [83, 86–88].

The catalytic properties of triphenylphosphine complex 79 and a series of pyridine analogs 80–85 derived from C,N-palladacycle based on 2-ferrocenylpyridine were assessed in the SM reaction between 4-iodotoluene and phenylboronic acid (Fig. 14, Scheme 23) [89]. In the reaction performed in air, most of pyridine complexes 80–85 appeared to be more effective (82–95%) than triphenylphosphine counterpart 79 (73%). However, in an inert atmosphere, the catalytic activity of complex 79 increased and the product yield reached 91%.

Scheme 23. Catalytic activity of complexes 79–85 in the SM reaction.

The authors noted that there is a linear correlation between the pKa value of the auxiliary pyridine ligands in complexes 80–85 and the yields of the coupling products. Complex 81, containing the strongest electron-donating ligand (X = Fc), afforded the lowest product yield (68%). In contrast, the highest product yield (95%) was observed in the reaction with CPC 80 bearing the weakest electron-donating auxiliary ligand (X = H).

7. Miscellaneous C,N-palladacycles

Recently, the catalytic activity in the Suzuki–Miyaura reaction of a series of C,N-palladacyclic derivatives based on phenyl- and naphthyloxazolines containing acenaphthoimidazol-2-ylidene as an auxiliary ligand was studied [90].

The most active (pre)catalyst was complex 86 with a bulky tert-butyl substituent at the fourth position of an oxazoline ring (Fig. 16). A wide range of (hetero)aryl bromides and (hetero)arylboronic acids were introduced into the SM reaction catalyzed by complex 86 to form biaryls in high yields (77–99%), albeit under severe temperature conditions. The biaryls containing polycyclic aromatic hydrocarbons, such as phenanthrene, anthracene, pyrene, and acridine, were also obtained in high yields (85–98%). Thus, the reaction of methoxy-substituted bromoquinoline with phenanthrenylboronic acid in the presence of complex 86 leads to a coupling product in high yield (Scheme 24).

Figure 16. Naphthyloxazoline and paracyclophane-based C,N-palladacycles.

Scheme 24. Catalytic activity of complex 86 in the SM reaction.

A large series of ternaphthalenes and diarylanthracenes were synthesized in high yields via the SM reaction catalyzed by complex 86. The advantages of this system include the use of equimolar amounts of organoboron compounds and aryl bromides, since an excess of the boron reagent is usually used in these reactions.

C,N-Palladacycle 87 featuring a paracyclophane backbone (Fig. 16) containing a 3-aminoimidazole moiety and triphenylphosphine as an additional ligand showed low catalytic activity in the SM cross-coupling [91]. Thus, in the SM reaction between various 4-halotoluenes and PhB(OH)₂ (Scheme 25), even at high temperatures, the coupling products were obtained in moderate yields (19–58%), depending on the nature of the halogen. As expected, the highest yield was achieved in the reaction with 4-iodotoluene (58%).

Scheme 25. Catalytic activity of complex 87 in the SM reaction.

The catalytic activity of both tetranuclear CPCs 88–90 based on thiosemicarbazone and their derivatives with bis(diphenylphosphino)methane (dppm) 91–96 (Fig. 17) was studied in the reaction of 4-bromoacetophenone with PhB(OH)₂ (Scheme 26) [92].

Figure 17. Thiosemicarbazone-based C,N-palladacycles.

Scheme 26. Catalytic activity of complexes 94–96 in the SM reaction.

The best conversions (96–98%) were achieved using binuclear CPCs 94–96. Under similar conditions, complexes 88–90 and 91–93 exhibited low catalytic activity (conversions 0–18% and 12–22%, respectively).

In contrast to the mentioned reports [47, 48, 50, 90], which described the catalytic activity of C,N-palladacycles with monodentate auxiliary carbene ligands, Fizia et al. [45] presented the results of testing in the Suzuki-Miyaura reaction of cyclopalladated carbene derivatives containing a pyrimidine backbone. Thus, mononuclear pyridine derivatives 97–100 (Fig. 18) exhibited high catalytic activity in the cross-coupling of aryl chlorides with arylboronic acids. In the case of aryl chlorides with electron-withdrawing substituents, high yields of the coupling products were achieved: for example, in the reaction of 4-chloroacetophenone with PhB(OH)₂ (1 mol %, K₃PO₄, ⁱPrOH/H₂O, 90 °C, 30 min), the product yields were 96–99%. In the case of aryl chlorides containing electron-donating substituents (-OMe), the reaction efficiency decreased markedly (39–53%). The yields drop significantly when sterically hindered arylboronic acids were introduced into the reaction. For example, in the reaction of 2-naphthylboronic acid with 4-methoxychlorobenzene, the yields of the coupling products were only 24–39%.

Figure 18. Cyclopalladated carbenes.

The catalytic activity of C,N-palladacycles 101–103 under similar conditions (1 mol %, K₃PO₄, ⁱPrOH/H₂O, 90 °C, 30 min) appeared to be extremely low: the yields of the coupling products were only 6–9%.

8. Conclusions

Hence, this literature survey provides information on the catalytic activity of various structural types of C,N-palladacycles in the Suzuki–Miyaura cross-coupling.

The analysis of the reported data showed that azapalladacycles are effective catalysts for the Suzuki–Miyaura reaction. CPCs exhibit high catalytic activity not only in the reactions of aryl iodides and aryl bromides with arylboronic acids, but also in the case of cheap and available aryl chlorides. In addition, the number of non-traditional halides (heterocycles, nucleosides) used in the reaction is increasing. It was also shown that the catalytic performance of CPCs depends on the metal surrounding and can vary depending on the nature of the auxiliary ligand and anion. Of particular note is a gradual transition from the model reactions to the synthesis of biologically relevant biaryls.

The main trends in recent years in the Suzuki–Miyaura cross-coupling with azapalladacycles are the following ones: a reduction in the catalyst loading and reaction temperature, a transition to an aqueous medium and other more environmentally benign solvents, the structural modification of palladacyclic catalysts by the immobilization on polymer substrates, which facilitates the process of their separation from the reaction mixture, and the use of IR and microwave activation as well as microfluidic technologies.

Abbreviations

Acknowledgements

This work was supported by the Ministry of Science and Higher Education of the Russian Federation (agreement no. 075-00277-24-00).

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